Perl 5.6 introduced something called interpreter threads. Interpreter threads are different from "5005threads" (the thread model of Perl 5.005) by creating a new perl interpreter per thread and not sharing any data or state between threads by default.

Prior to perl 5.8 this has only been available to people embedding perl and for emulating fork() on windows.

The threads API is loosely based on the old Thread.pm API. It is very important to note that variables are not shared between threads, all variables are per default thread local. To use shared variables one must use threads::shared.

It is also important to note that you must enable threads by doing use threads as early as possible in the script itself and that it is not possible to enable threading inside an eval "", do, require, or use. In particular, if you are intending to share variables with threads::shared, you must use threads before you use threads::shared and threads will emit a warning if you do it the other way around.

This will wait for the corresponding thread to join. When the thread finishes, join() will return the return values of the entry point function. If the thread has been detached, an error will be thrown.

The context (scalar or list) of the thread creation is also the context for join(). This means that if you intend to return an array from a thread, you must use my ($thread) = threads-new(...)>, and that if you intend to return a scalar, you must use my $thread = ....

If the program exits without all other threads having been either joined or detached, then a warning will be issued. (A program exits either because one of its threads explicitly calls exit(), or in the case of the main thread, reaches the end of the main program file.)

This will return the id of the thread. Thread IDs are integers, with the main thread in a program being 0. Currently Perl assigns a unique tid to every thread ever created in your program, assigning the first thread to be created a tid of 1, and increasing the tid by 1 for each new thread that's created.

NB the class method threads->tid() is a quick way to get the current thread id if you don't have your thread object handy.

async creates a thread to execute the block immediately following it. This block is treated as an anonymous sub, and so must have a semi-colon after the closing brace. Like threads->new, async returns a thread object.

A thread (not necessarily the main thread) exited while there were still other threads running. Usually it's a good idea to first collect the return values of the created threads by joining them, and only then exit from the main thread.

The current implementation of threads has been an attempt to get a correct threading system working that could be built on, and optimized, in newer versions of perl.

Currently the overhead of creating a thread is rather large, also the cost of returning values can be large. These are areas were there most likely will be work done to optimize what data that needs to be cloned.

Creating threads inside BEGIN blocks (or during the compilation phase in general) does not work. (In Windows, trying to use fork() inside BEGIN blocks is an equally losing proposition, since it has been implemented in very much the same way as threads.)